204 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 



fluence that any given degree of temperature exerts 

 on the rate of the natural and of the artificial 

 rhythm respectively. Further, it will be remembered 

 that in warm water the natural rhythm, besides 

 being quicker, is not so regular as it is in cold 

 water; thus also it is with the artificial rhythm. 

 Again, water below 20° or above 85° suspends the 

 natural rhythm, i.e. stops the contractions; and 

 the artificial rhythm is suspended at about the 

 same degrees. Lastly, just as there are considerable 

 individual variations in the extent to which the 

 natural rhythm is affected by temperature, so the 

 artificial rhythm is in some cases more influenced by 

 this cause than in others, though in all cases it 

 further resembles the natural rhythm in showing 

 some considerable degree of modification under 

 such influence. 



On the whole, then, it would be impossible to 

 imagine two cases more completely parallel than 

 are these of the effects of temperature on natural 

 and on artificial rhythm respectively; and as it 

 must be considered in the last degree improbable 

 that all these coincidences are accidental, I conclude 

 that the effects of temperature on the natural 

 rhythm of Medusae (and so, in all probability, on 

 the natural rhythm of other ganglio-muscular 

 tissues) are for the most part exerted, not on the 

 ffanHionic, but on the contractile element. 



In order to test the effects of gases on the 

 artificial rhytlim, I took a severed quadrant of 

 Aurelia, and floated it in sea-water, with its 

 muscular surface just above the level of the water. 



